1. Field of the Invention
The present invention relates to an image forming system that forms images on both sides of a recording medium using two image forming apparatuses.
2. Description of the Background
As an image forming system in which images are formed on both sides of a recording medium (typically paper), an image forming system having two printing apparatuses arranged in series along the conveyance path of the paper is known, as disclosed, for example, in Japanese patent application publication no. H07-237336(JP-H07-237336-A). In this image forming system, the first, upstream image forming apparatus forms an image on the first side of a long recording medium longer in the conveyance direction, the long recording medium discharged from the first image forming apparatus is reversed by a reversing device, and the second, downstream image forming apparatus forms an image on the second (reverse) side of the long recording medium.
In the image forming system described above, for example, if the first image forming apparatus forms images by electrophotography, the recording medium (e.g., paper) may expand or contract due to heating in the heat fixing process, in which the toner image transferred to the recording medium is melted and fixed on the recording medium.
When the recording medium expands or contracts, the length of the recording medium when it is fed into the second image forming apparatus changes from the original length so that the first page of the recording medium has a different length from the second page. Consequently, it can happen that the position of the image formed on the first side does not align with that on the second page.
In addition, when the first image forming apparatus forms images using the inkjet system, the recording medium may expand or contract due to heat applied thereto in the drying process after discharging ink to form an image on the recording medium. Therefore, if the first image forming apparatus forms images using the ink jet system, the same positioning problem arises as in the case in which the first image forming apparatus forms images by electrophotography.
To solve the problem described above, for example, JP-H07-237336-A describes an image forming system that aligns the image position on the first side of a recording medium and the image position on the second side thereof by forming alignment marks at predetermined positions (e.g., at the top of a page) on the recording medium with the first image forming apparatus, measuring the distance between the alignment marks or the detection timing thereof with the second image forming apparatus, and changing the conveyance speed of the recording medium based on the measuring result.
However, although the technologies described above are successful in that they do align the image positions along the conveyance direction of the recording medium (hereinafter referred to as the sub-scanning direction), they are not capable of making alignment of the image positions in the direction perpendicular to the recording medium (hereinafter referred to as the main scanning direction). That is, expansion and contraction of the recording medium caused by heating may occur not only in the sub-scanning direction but also in the main scanning direction depending on the type of paper. Herein, “different types of paper” means, for example, paper having different characteristics such as length of a page formed on the recording medium and the distance between the alignment marks on the recording medium, in addition to different dimensions such as the thickness and the width (length along the main scanning direction), the material, etc. of the recording medium. If the recording medium expands or contracts in the main scanning direction as well as the sub-scanning direction, the image positions of the first side and the second side of the recording medium are not aligned properly by the technologies described above.